The present invention relates to a sheet stacking apparatus installed in an image forming apparatus such as a printer or copier, and particularly relates to a sheet stacking apparatus equipped with a sheet support tray for temporarily holding a sheet when a finishing process such as stapling, punching and marking is performed on the sheet with an image formed thereon, and an image forming apparatus equipped with the sheet stacking apparatus.
Generally, a sheet stacking apparatus is installed at a final processing portion of a device such as a printer, copier or facsimile machine for sequentially stacking and storing sheets processed by an image forming apparatus. The sheet stacking apparatus includes one type in which a pair of transport rollers nips a sheet to transport to a stacker, and another type in which a push-out member pushes a trailing edge of a sheet to transport to a stacker after the sheet is temporarily stacked on a support tray.
The invention relates to the latter type, i.e. a sheet pushing mechanism moves a sheet from a tray for temporarily supporting the sheet to a stacker for sequentially stacking the sheet. The invention also relates to a mechanism for aligning the sheet on the support tray for temporarily supporting the sheet and for securely discharging the sheet from the tray.
Japanese Patent Publication (Kokai) No. 2002-060118 has disclosed a conventional sheet pushing mechanism in which a predetermined number of sheets are stacked on a tray for temporarily supporting the sheets, and a push-out member protruding above the tray pushes a bundle of the sheets toward a stacker. The tray is provided with a long groove at a center portion thereof extending from a side that the sheet is transported to a side that the sheet is discharged. The push-out member is arranged to protrude above the tray from a backside thereof. The push-out member has a base portion attached to an endless belt disposed at the backside of the tray. When a pulley of the endless belt rotates, the push-out member moves from a trailing end to a leading end on the tray. Accordingly, the push-out member revolves around the tray together with the endless belt disposed at the backside of the tray, so that the push-out member stands upward above the tray in a half rotation of the endless belt, and revolves downwardly at the backside of the tray in the other half rotation of the endless belt.
Japanese Patent Publication (Kokai) No. 2002-179322 has disclosed a similar structure, in which a protruding push-out member revolves around an endless belt. In such a structure, it is necessary to provide a large space for the push-out member, and it is difficult to arrange a component such as a sensor at the backside of the tray, thereby increasing a size of the apparatus.
In the conventional finishing apparatus, it is necessary to temporarily store sheets from an image forming apparatus on a tray for finishing, and transport the finished sheets to a discharge stacker. The tray for temporarily stacking the sheets is provided with aligning means for aligning the sheets at a predetermined position for finishing, and transporting means for transporting the finished sheets (single sheet or a bundle of sheets) to a discharge tray.
In general, the aligning means includes a pair of aligning plates arranged on left and right sides for pushing sides of the sheets to align in a width direction. When the center of the sheet is used as a reference, the aligning plates move to the center. When a side edge of the sheet is used as reference, one of the aligning plates is fixed, and the other of the aligning plates moves. The transporting means includes a pair of rollers or an endless belt rotatably disposed on the stacker, or a sheet pushing member disposed on the tray to be reciprocally movable for pushing the trailing edge of the sheets. Particularly, when the sheets are stapled into a bundle, the sheet pushing member is preferably used.
The sheets are stacked on the tray, and the aligning plates abut against the sheets for alignment as described above. After the sheets are finished (stapled), the sheet pushing member (protruding portion) moves the sheets to fall into a discharge stacker for storage. As a method of aligning the sheet, it is known that a tray curved in a direction perpendicular to the aligning direction aligns the sheets using stiffness of the sheets, thereby preventing the sheets from being curled and disorganized. It is also known that protruding guide means such as a rib is disposed in the tray, so that when the sheets are discharged, the sheets are forcibly bent in a direction perpendicular to the transport direction for securely transporting the sheets.
When the sheets are temporarily stacked on the tray, the sheets are aligned in a direction perpendicular to the transport direction. In the case that the tray is curved to correctly align the curled or thin sheets, when the sheets are transported, the sheets tend to bend, thereby causing a transport problem or the trailing edge of the sheet to remain on the tray. In the case that the protruding guide means such as a rib is disposed in the tray for securely transporting the sheets, the sheets tend to bend and it is difficult to correctly align the sheets. Therefore, it is necessary to provide a flat tray for, temporary stacking the sheets even in a case that a curled sheet or a thin sheet is processed in the image forming apparatus. Also, it is necessary to discharge the sheets from the tray in a flat state, thereby causing a problem in aligning and discharging the sheets.
In view of the problems described above, a first object of the present invention is to provide a compact and simple sheet stacking apparatus with low cost, while solving the problems in which the protruding member revolves around the endless belt and the apparatus needs a large space.
A second object of the present invention is to provide a sheet stacking apparatus in which a pushing member is not caught by a sheet when the pushing member moves at a backside of a tray after the pushing member pushes a trailing edge of the sheet to the stacker and passes the sheet stacked on a stacker, even when an excessive amount of sheets is stacked on the stacker or the sheets are curled.
A third object of the present invention is to provide a sheet stacking apparatus in which a pushing member pushes the sheets on a stacker to align the sheets after the pushing member transports the sheets to the stacker.
A fourth object of the present invention is to provide a sheet stacking apparatus in which it is possible to discharge the sheets from a stacker and securely align the sheets without negative effect on the alignment.
Further objects and advantages of the invention will be apparent from the following description of the invention.